This invention relates to the recovery of precious metals from metal alloys produced aluminothermically from base metal smelter slag.
U.S. Pat. No. 5,865,872 (Krofchak et al) describes a method of recovering metals and producing a secondary slag from base metal smelter slag by means of an aluminothermic reaction. In addition to the secondary slag, the reaction produces a metal alloy containing copper, nickel and/or cobalt, iron and sulphur. The contents of the aforementioned U.S. patent is hereby incorporated herein by reference. A base metal smelter slag may include (by weight) the following:
from about 10% to about 50% SiO2,
from about 0.5% to about 10% Al2O3,
from about 25% to about 50% Fe,
from about 0.5% to about 20% CaO,
from about 0.5% to about 15% MgO,
from about 0.1% to about 15% Cu,
from about 0.1% to about 15% Ni,
from about 0.1% to about 7% Co,
from about 0.1% to about 3% S
A metal alloy produced in accordance with the method described in the aforementioned patent may include (by weight):
from about 2.5% to about 99% Fe.
from about 0.1% to about 10% Si,
from about 0.1% to about 25% Ni,
from about 0.2% to about 25% Cu.
from about 0.1% to about 10% Co,
from about 0.1% to about 25% S
A base metal smelter slag may also include precious metals as follows (by weight):
from about 0.01 oz/ton to about 5 oz/ton Ag,
from about 0.01 oz/ton to about 3 oz/ton Pt,
from about 0.01 oz/ton to about 3 oz/ton Pd,
from about 0.01 oz/ton to about 3 oz/ton Au,
from about 0.01 oz/ton to about 3 oz/ton Rh
Accordingly, a metal alloy produced from a base metal smelter slag containing precious metals in accordance with the method described in the aforementioned patent may also contain precious metals (by weight) as follows:
from about 0.02 oz/ton to about 10 oz/ton Ag,
from about 0.02 oz/ton to about 6 oz/ton Pt,
from about 0.02 oz/ton to about 6 oz/ton Pd.
from about 0.02 oz/ton to about 6 oz/ton Au,
from about 0.02 oz/ton to about 6oz/ton Rh
It is therefore an object of the invention to provide a method of recovering at least one precious metal from a metal alloy produced aluminothermically from a base metal smelter slag.
The preset invention is based on the discovery that, if such a metal alloy is treated with aqueous sulphuric acid solution having a pH which does not exceed about 2, nickel and/or cobalt and iron is dissolved with the formation of hydrogen sulphide which reacts with copper in the metal alloy to form a copper sulphide precipitate which contains at least one precious metal, so that the precipitate can be easily separated from the remaining solution which contains cobalt sulphate and/or nickel sulphate and ferrous sulphate. The copper sulphide precipitate containing the at least one precious metal can then be treated to recover the precious metal.
The remaining solution may be subjected to an evaporation step to cause crystallizing of ferrous sulphate as a hydrate, and removing the crystallized ferrous sulphate therefrom to produce a further remaining solution containing cobalt sulphate and/nickel sulphate and any remaining ferrous sulphate.
The pH of the further remaining solution may be raised to about 3.5 and converting the ferrous iron to ferric iron to cause precipitation of iron as ferric hydroxide and removing the precipitated ferric hydroxide from the cobalt sulphate and/or nickel sulphate containing solution.
The pH of the nickel sulphate and/or cobalt sulphate containing solution may be raised to at least 9 with sodium carbonate and/or nickel sulphate containing solution to at least about 9 with sodium carbonate to precipitate cobalt sulphate and/or nickel sulphate as cobalt carbonate and/or nickel carbonate, and removing the precipitated cobalt carbonate and/or nickel carbonate from the still remaining solution.